In order to enhance running stability of an vehicle, it has been required to estimate a friction coefficient (a road friction coefficient) between a tire and a road or contact state of the tire accurately and then feed it back to a vehicle controller. If the road friction coefficient or contact state of the tire can be estimated in advance, that allows advanced braking by an ABS brake, before operation for danger avoidance such as braking or steering is carried out. It would lead to enhance safety of the vehicle moreover.
As the method of estimating the road friction coefficient, for example, there has been provided that the method comprising steps of detecting wheel speed, detecting a wheel speed fluctuation ω when disturbance T is suffered in accordance with said detected wheel speed signal ω, identifying a transfer function of the wheel as satisfied with ω by the least-square method, estimating inclination of the road μ, estimating braking force of the vehicle in accordance with relationship between inclination μ and braking force obtained in advance, estimating inclination of the road μ when a slip rate is zero with accordance with said braking force and inclination of the road μ (refer to the Patent Document 1). Also, as shown in FIG. 9, there has been provided that the method of estimating road friction coefficient comprising steps of forming a sensing block 52H which are higher than a surface of a tread and a sensing block 52L which are lower than the surface of the tread on a tire tread portion 51 of a pneumatic tire 50, sticking a strain gauge 53H and 53L to a side surface which is parallel to each sensing block in a tire circumference direction and estimating road friction coefficient in accordance with a strain level difference detected by the two strain gauge 53H and 53L and a map indicating relationship between the strain level and the road friction coefficient which are obtained in advance (refer to the Patent Document 2).    Patent Document 1: Japanese Unexamined Patent Application Publication 2002-160620    Patent Document 2: Japanese Unexamined Patent Application Publication 2002-36836